In lithography, there is an ongoing desire to reduce the size of features in a lithographic pattern to increase the density of features on a given substrate area. In photolithography, the push for smaller features has resulted in the development of technologies such as immersion lithography and extreme ultraviolet (EUV) lithography, which are however rather costly.
A potentially less costly road to smaller features that has gained increasing interest is so-called imprint lithography, which generally involves the use of a template to transfer a pattern onto a substrate. An advantage of imprint lithography is that the resolution of the features is not limited by, e.g., the wavelength of a radiation beam or the numerical aperture of a projection system as in photolithography, but mainly just by the pattern density on the template (also referred to as a stamp). There are three main approaches to imprint lithography, examples of which are schematically depicted in FIGS. 1a to 1c. 
FIG. 1a shows an example of a type of imprint lithography that is often referred to as micro-contact printing. Micro-contact printing involves transferring a layer of molecules 11 (typically an ink such as a thiol) from a template 10 (e.g. a polydimethylsiloxane (PDMS) template) onto a resist layer 13 which is supported by a substrate 12 and planarization and transfer layer 12′. The template 10 has a pattern of features on its surface, the molecular layer being disposed upon the features. When the template is pressed against the resist layer, the layer of molecules 11 are transferred onto the resist. After removal of the template, the resist is etched such that the areas of the resist not covered by the transferred molecular layer are etched down to the substrate. For more information on micro-contact printing, see e.g. U.S. Pat. No. 6,180,239.
FIG. 1b shows an example of so-called hot imprint lithography (or hot embossing). In a typical hot imprint process, a template 14 is imprinted into a thermosetting or a thermoplastic polymer resin 15, which has been cast on the surface of a substrate 12. The resin may, for instance, be spin coated and baked onto the substrate surface or, as in the example illustrated, onto a planarization and transfer layer 12′. When a thermosetting polymer resin is used, the resin is heated to a temperature such that, upon contact with the template, the resin is sufficiently flowable to flow into the pattern features defined on the template. The temperature of the resin is then increased to thermally cure (crosslink) the resin so that it solidifies and irreversibly adopts the desired pattern. The template may then be removed and the patterned resin cooled. In hot imprint lithography employing a layer of thermoplastic polymer resin, the thermoplastic resin is heated so that it is in a freely flowable state immediately prior to imprinting with the template. It may be necessary to heat a thermoplastic resin to a temperature considerably above the glass transition temperature of the resin. The template is pressed into the flowable resin and then cooled to below its glass transition temperature with the template in place to harden the pattern. Thereafter, the template is removed. The pattern will comprise the features in relief from a residual layer of the resin which may then be removed by an appropriate etch process to leave only the pattern features. Examples of thermoplastic polymer resins used in hot imprint lithography processes are poly (methyl methacrylate), polystyrene, poly (benzyl methacrylate) or poly (cyclohexyl methacrylate). For more information on hot imprint, see e.g. U.S. Pat. Nos. 4,731,155 and 5,772,905.
FIG. 1c shows an example of UV imprint lithography, which involves the use of a transparent template and a UV-curable liquid as resist (the term “UV” is used here for convenience but should be interpreted as including any suitable actinic radiation for curing the resist). A UV curable liquid is often less viscous than a thermosetting or thermoplastic resin used in hot imprint lithography and consequently may move much faster to fill template pattern features. A quartz template 16 is applied to a UV-curable resin 17 in a similar manner to the process of FIG. 1b. However, instead of using heat or temperature cycling as in hot imprint, the pattern is “frozen” by curing the resin with UV radiation that is applied through the template onto the resin. After removal of the template, the pattern will comprise the features in relief from a residual layer of the resin which may then be removed by an appropriate etch process to leave only the pattern features. A particular manner of patterning a substrate through UV imprint lithography is so-called step and flash imprint lithography (SFIL), which may be used to pattern a substrate in a number of subsequent steps in a similar manner to optical steppers conventionally used in, e.g., IC manufacture. For more information on UV imprint, see e.g. U.S. patent application publication no. 2004-0124566, U.S. Pat. No. 6,334,960, PCT patent application publication no. WO 02/067055, and the article by J. Haisma entitled “Mold-assisted nanolithography: A process for reliable pattern replication”, J. Vac. Sci. Technol. B 14(6), November/December 1996.
Combinations of the above imprint techniques are also possible. See, e.g., U.S. patent application publication no. 2005-0274693, which mentions a combination of heating and UV curing a resist.
In the above mentioned approaches to imprint lithography, especially hot imprint lithography and UV imprint lithography, an imprintable material (e.g., imprint liquid) is sandwiched between a substrate and a template to form a thin continuous layer over the entire area of the template. When the imprintable material is sandwiched between the substrate and the template, the material is expelled outwardly to, ultimately, form a thin continuous layer of material. At the same time, gas (e.g., air) present between the template and the substrate is also sandwiched. As the template is comes closer to the substrate it is more difficult for the imprintable material and gas to flow outwardly and around features of the template. The closer the template is to the substrate, the more slowly the imprintable material flows. Thus, since the imprintable material flows slowly, the time taken to form a thin continuous layer increases. Similarly, the closer the template is to the substrate, the longer it takes to expel gas from between the imprint template and the substrate. It is also possible that gas sandwiched between the template and the substrate may become trapped, forming gas bubbles which introduce defects into the imprint pattern.